Viscosity Measurement Tube

Quick Answer: The Viscosity Measurement Tube is a set of tall transparent tubes held in a stand, used for falling-ball experiments. Students drop a small ball into each liquid, time its steady fall between two marks and compare or calculate the viscosities using Stokes’ law.

The Falling-Ball Method

The listing image shows the viscosity measurement tube as three clear tubes held upright in a stand on a base, each filled with a different coloured liquid and each with a small ball. Viscosity is a liquid’s resistance to flow, and a sphere falling through it feels a drag force that grows with its speed. Soon after release the ball reaches terminal velocity, where the drag and the upthrust together balance its weight, and from then on it falls at constant speed.

That constant speed is what students measure. Two marks are placed on the tube well below the surface, so that the ball has reached terminal velocity before the first, and the time to fall between them is recorded. For slow, steady fall in a wide tube, Stokes’ law gives the viscosity as η = 2r²(ρs – ρl)g / 9v, where r is the ball’s radius, ρs and ρl are the densities of ball and liquid, and v is the terminal velocity.

Three tubes side by side make comparison quick: a ball drops through water in a moment, while in glycerol or a thick oil it sinks slowly enough to time easily. Temperature has a large effect, since most liquids become much less viscous as they warm. Which liquids and balls are supplied, and the tube dimensions, should be confirmed at enquiry.

Specifications

Practical Work

• Ranking liquids by viscosity from the fall times of identical balls
• Measuring terminal velocity and calculating viscosity with Stokes’ law
• Investigating how ball size affects terminal velocity in one liquid
• Showing how warming a liquid reduces its viscosity

Care & Handling

• Retrieve steel balls with a magnet, or other balls with a long-handled scoop, rather than tipping the tubes.
• Keep viscous liquids covered between lessons to stop dust falling in and moisture being absorbed from the air.
• Wash the tubes with warm water and detergent when changing liquids, and dry them fully.
• Stand the viscosity measurement tube on a level bench so the balls fall along the tube axis.

Why Choose LabEquip

Physics teachers need a viscosity practical that gives measurable times, and a multi-tube stand lets groups compare liquids without setting up separate columns. LabEquip lists this apparatus in the STEM kits range, next to the Bernoulli Equation Verification Apparatus for the wider fluid mechanics topic. Send your enquiry through the contact page.

Frequently Asked Questions

Why must the first timing mark be below the liquid surface?

Right after release the ball is still speeding up. It needs a short distance to reach terminal velocity, when the forces balance. Starting the timing too high gives a speed that is too low and a viscosity value that is too high.

What is terminal velocity?

It is the steady speed reached when the upward drag and upthrust on the ball exactly balance its weight. The resultant force is then zero, so the ball stops accelerating and falls at a constant speed.

Which liquids work well in the tube?

Glycerol, castor oil, liquid paraffin and washing-up liquid give slow, easily timed falls. Water is useful as a comparison, but a ball drops through it too quickly for accurate timing. Label each tube with its contents.

Why does temperature affect the result so much?

The molecules of a warmer liquid move more freely and slide past each other more easily, so the liquid flows more readily. The viscosity of glycerol, for example, falls steeply as it warms, so record the room temperature with each result.

Does the tube width matter?

Yes. Stokes’ law assumes the liquid extends a long way around the ball. In a narrow tube the walls add extra drag and slow the ball, so the ball should be small compared with the tube diameter, or a wall correction should be applied.

How are the balls recovered after a run?

Steel balls can be drawn up by sliding a magnet along the outside of the tube or lifted with a magnet on a rod. Other balls are retrieved with a small scoop or by draining the tube into a clean container, so the liquid can be reused.

Last Updated: September 2026